In addition to the combat deck a mage can have up to three extra cards:
— Guild card (center);
— Alliance card (left);
— Goblin’s card (right).

Guild card has its dedicated article available here.

Alliance card is a guild card of your ally guild. If you haven't got an ally there is no alliance card either (makes sense). Details are in Guild Alliances.

— Goblin’s card is a temporary card and can be obtained for the duration of one week only. (Good thing is you can get it every week).
— Goblin’s card is inactive in guild raids and the campaign.
— If Goblin’s card is weaker than your ninth combat card then it only increases your health.
— If Goblin’s card possesses more power than your ninth combat card then it not only adds to your health but also acts as an extra card in your combat deck.
— Goblin’s card can be obtained on Mondays only.
— Card’s power depends on your current Dueling league.
— This card is of random element. Nobody knows what element you will get this week.
— The card can be obtained either by buying gold (min 100 at once) or by suborning the goblin with 7 000 000 (seven million silver). There are no other ways.
— A mage can get additional bonuses along with the card (gold to the treasury, silver and experience buffs) if he/she buys 1000 gold or more at once.
— Attention! Every Monday one can get the card and bonuses only once. All following purchases come with the standard bonuses only.